1. Field of the Invention
The present invention relates generally to relays for interrupting power on power conductors upon occurrence of a fault condition, such as a phase loss, ground fault, overload, or undercurrent condition. More particularly, the present invention relates to a relay that can be selectively configured to operate in either a single-phase mode or a multi-phase mode and which determines and reports diagnostic parameters associated with current flow through the power conductors. The relay is further configured to provide protection for fault conditions based on the determined diagnostic parameters, regardless of the selected mode of operation.
2. Description of the Related Art
Single-phase and multi-phase (e.g., three-phase) power systems typically include an overload relay for interrupting power in the power conductors when a fault condition occurs, such as a ground fault, phase loss, overcurrent, or undercurrent condition. A variety of types of overload relays are available, ranging from simple big-metal or eutectic overload relays to more complex, solid-state relays which may include some intelligence and/or reporting capabilities. Big-metal and eutectic overload relays include heater elements in each phase which open when an excessive current flowing through the heater elements causes the element to exceed a specific temperature. Solid-state relays, on the other hand, include electronic devices for monitoring phase current and for determining, based on the monitored current, whether a fault condition has occurred. Thus, solid-state relays typically can be configured to provide protection for ground fault, undercurrent and phase loss conditions, in addition to overcurrent conditions.
To provide such protection, however, the electronic devices included in a solid-state relay require power for proper operation. Such power may be provided externally from a separate source, or, the relay may be self-powered, meaning the power for the electronic devices is derived from an internal source, such as the relay""s current transformers which are monitoring the current in each phase. The solid-state relay may also be configured to include reporting capabilities. For example, such a relay may communicate diagnostic information, such as an average current in the power conductors or a percentage current imbalance between the conductors.
Both big-metal/eutectic relays and solid-state relays are available in a single-phase configuration and a three-phase configuration. A typical overload relay configuration for a three-phase application is illustrated in FIG. 1, and a typical overload relay configuration for a single-phase application is illustrated in FIG. 2.
FIG. 1 illustrates the conventional use of an overload relay 16 in a three-phase application. In FIG. 1, three-phase power conductors 10a, 10b and 10c are connected to a motor 11 through short-circuit protection devices 12a, 12b, and 12c (e.g., circuit breakers, fuses, etc.), a contactor 14 (including contact pairs 14a/axe2x80x2, 14b/bxe2x80x2, and 14c/cxe2x80x2), and an overload relay 16 (including relay paths 16a, 16b, and 16c), as shown. Relay xe2x80x9cpathsxe2x80x9d 16a, 16b, and 16c may be the heater elements of a big-metal or eutectic relay which are in series with the power conductors and open to interrupt current flow through the power conductors upon occurrence of an overcurrent condition. Alternatively, paths 16a-c may simply be pass-through conductors through which the phase currents flow through relay 16 and on which phase currents are monitored. In such a device, relay 16 interrupts current flow upon detection of a fault condition by generating a trip signal which, in turn, causes an interruption in current flow through the power conductors. For example, such a trip signal may be used to de-energize the coil in a contactor (such as the coil of contactor 14), which results in opening of contactor pairs (e.g., pairs 14a/axe2x80x2, 14b/bxe2x80x2, and 14c/cxe2x80x2) connected in series with the power conductors. The designations xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, and xe2x80x9ccxe2x80x9d are used herein to identify elements associated with phase xe2x80x9caxe2x80x9d, phase xe2x80x9cbxe2x80x9d, and phase xe2x80x9ccxe2x80x9d of the single-phase or multi-phase system.
In FIG. 2, overload relay 16 is configured for use in a single-phase application in which current is conducted through power conductors 10a and 10b (i.e., phase xe2x80x9caxe2x80x9d current and phase xe2x80x9cbxe2x80x9d current). As shown, the components have been wired such that motor 11 is connected only to overload relay paths 16a and 16c. The phase xe2x80x9ccxe2x80x9d load current provided to motor 11 is routed through overload relay path 16b and contactor pair 14b/bxe2x80x2, and then through overload relay path 16c, contactor pair 14c/cxe2x80x2, and short circuit protection device 12b (i.e., the phase xe2x80x9cbxe2x80x9d components are connected in series with the phase xe2x80x9ccxe2x80x9d components).
Proper operation of the overload relay 16 requires that the phase xe2x80x9cbxe2x80x9d current must be routed through both the phase xe2x80x9cbxe2x80x9d components and the phase xe2x80x9ccxe2x80x9d components, even though such a configuration results in extra wiring costs (as well as other drawbacks which will be explained below). For example, if overload relay 16 is a big-metal or eutectic overload relay, load current must be routed through all three heater elements to ensure accurate overload trip protection. Otherwise, special calibration or adjustments must be performed such that the big-metal or eutectic relay will operate properly. If relay 16 is a self-powered solid-state overload relay, current may need to flow through the current transformer in all three phases such that the current transformers can provide sufficient energy to power the relay""s electronics. Further, if a self-powered or externally-powered overload relay is to provide phase loss protection, current must be routed through all three conductors to prevent an improper phase loss indication. That is, an apparent current imbalance would be indicated if phase loss protection is enabled and current is not routed through one of the three phases. Still further, a solid-state overload relay with a reporting feature will inaccurately calculate and report average current and current imbalance if current is not routed through each of the phase xe2x80x9caxe2x80x9d, phase xe2x80x9cbxe2x80x9d, and phase xe2x80x9ccxe2x80x9d conductors as shown in FIG. 2.
Although the configuration illustrated in FIG. 2 resolves many of the problems that arise when using an overload relay in a single-phase application, problems still remain. In particular, a solid-state overload relay used in a single-phase system configured in accordance with FIG. 2 cannot provide ground fault protection. Three-phase solid-state relays typically detect the occurrence of a ground fault in a three-phase system by monitoring or determining the vector sum of the currents in each phase. Normal operation is indicated when the phase currents substantially cancel, and a ground fault is indicated if the vector sum of the phase currents exceeds a predetermined threshold value. If, however, such an overload relay is used in a single-phase application and configured as shown in FIG. 2, the vector sum of the phase currents would be equivalent to the magnitude of the single-phase current (i.e., the vector currents through phase xe2x80x9caxe2x80x9d and phase xe2x80x9cbxe2x80x9d would cancel such that the resulting vector sum would be the current through phase xe2x80x9ccxe2x80x9d), resulting in inaccurate determination of the vector sum and improper indication of a ground fault condition.
To avoid the loss of ground fault protection when using a solid-state overload relay in a single-phase application, the system can be configured as shown in FIG. 3. In FIG. 3, the load current for conductor 12a (i.e., phase xe2x80x9caxe2x80x9d) is routed to the motor through the phase xe2x80x9caxe2x80x9d components (i.e., short circuit protection device 12a, contactor pair 14a/axe2x80x2, and relay path 16a). Similarly, the load current for conductor 10b (i.e., phase xe2x80x9cbxe2x80x9d) is routed to the motor through the phase xe2x80x9cbxe2x80x9d components (i.e., short circuit protection device 12b, contactor pair 14b/bxe2x80x2, and relay path 16b). Conductor 10c and the phase xe2x80x9ccxe2x80x9d components (i.e., relay path 16c, contactor pair 14c/cxe2x80x2, and short circuit protection device 12c) simply are not connected in the single-phase application. Although the configuration illustrated in FIG. 3 resolves the ground fault protection problem, it reintroduces the problems associated with accurate detection of overcurrents when using bi-metal/eutectic overload relays, insufficient supply of energy when using self-powered overload relays, and inaccurate reporting of current-related parameters and detection of underload and current imbalance conditions when using solid-state overload relays.
Accordingly, although an overload relay may be configured for both single-phase and three-phase applications, such interchangeable use has its drawbacks. Most particularly, an overload relay configured for a single-phase application can provide phase loss protection and accurate reporting of average current and current imbalance if configured in accordance with FIG. 2, but at the expense of ground fault protection. On the other hand, ground fault protection is provided if the relay is configured in accordance with FIG. 3, but underload and current imbalance protection and reporting capabilities are compromised.
It would be desirable, therefore, to provide a versatile, configurable overload relay that could be used in both single-phase and multi-phase applications. Such a relay would provide protection from overcurrent, undercurrent, current imbalance, phase loss, and ground faults in both the single-phase mode and the multi-phase mode of operation. Further, if the overload relay includes reporting capabilities, the relay would accurately report diagnostic information, such as average current and current imbalance, regardless of the mode of operation. Further still, to reduce wiring costs associated with using the overload relay in a single-phase application, the relay would preferably be configured as illustrated in FIG. 3, in which one of the three conductors and the associated phase components simply are not connected.
The present invention provides a relay which offers the aforementioned capabilities. The relay is configurable such that it operates in either a single-phase mode or a multi-phase mode.
Thus, in accordance with one aspect of the invention, a configurable relay for interrupting power provided by a plurality of power conductors includes an input configured to receive a selection parameter to select a single-phase or a multi-phase mode of operation, a sensor circuit to monitor the current flow through the power conductors, a control circuit, and an output. The control circuit, which is in communication with the input and the sensor circuit, is configured to determine a diagnostic parameter associated with the current flow based on the mode that is selected and the output indication from the sensor circuit. The output, which is in communication with the control circuit, provides an output signal based on the diagnostic parameter. The diagnostic parameter may be, for example, average current. The output signal, for example, may include a reporting signal representative of the magnitude of the average current or may include a trip signal to interrupt the current flow through the power conductors based on the diagnostic parameter.
In accordance with another aspect of the invention, a configurable relay for interrupting current flow through a plurality of power conductors configured for a single-phase application or a multi-phase application includes an input configured to receive a selection parameter to select the mode of operation, a plurality of sensors configured to monitor current flow in the power conductors, a control circuit in communication with the input and the sensors, and an output in communication with the control circuit. The sensors provide sensor output signals representative of the current in the power conductors and the vector sum of currents in the power conductors. The control circuit is configured to determine the occurrence of a ground fault condition and a phase loss condition based on the selected mode and the sensor output signals. The output provides an output signal upon occurrence of the ground fault condition and upon occurrence of the phase loss condition.
In accordance with yet another aspect of the invention, a configurable relay for monitoring parameters associated with power provided by a plurality of power conductors includes an input configured to receive a selection signal representative of a selection of one of a single-phase or multi-phase mode of operation, a plurality of sensors to monitor current in the power conductors, a control circuit in communication with the input and the sensors, and an output in communication with the control circuit. The control circuit is configured to determine a parameter associated with the current in the power conductors. If the single-phase mode of operation has been selected, then the control circuit determines the parameter based on sensor output signals representative of the current in two of the power conductors. If the multi-phase mode of operation has been selected, then the control circuit determines the parameter based on sensor output signals representative of the current in at least three of the power conductors. The output provides an output signal that is representative of the determined parameter.